1. Field of the Invention.
This invention relates to a method of evaluating the quality of a torquing operation in which a rotary tool exerts force on a workpiece such as a threaded fastener and to apparatus for measuring the torquing operation to develop quantitative signals which may be used in the evaluation method.
2. Prior Art
In order to insure the proper setting of threaded fasteners such as nuts and bolts, it is common practice to establish a value for the maximum torque to be applied to the fastener by a torquing tool and to continue the torquing operation until this maximum value has been attained. Spring-loaded torque gages are available for measuring the torque imposed manually, with a wrench, and a variety of torque limiting devices are available for use on automatic equipment such as the hydraulic nut drivers commonly used in automobile assembly and the like. Torque limiting devices often include a hydraulic pressure limiting mechanism or a mechanical slip clutch to limit the torque. In other systems the torque is electronically monitored using a strain gage of other suitable force transducer and the driving power is electrically terminated when the predetermined maximum torque is achieved.
However, the imposition of a maximum predetermined torque on a threaded fastener does not in itself guarantee the quality of the fastener setting. For example, if the threads are improperly mated; i.e. crossed, at the outset of the torquing operation, the maximum torque may be quickly attained without any real closure of the fastener. If the instantaneous torque exerted on the fastener is plotted against the time of the torquing operation the curve for this crossed thread situation will be practically a vertical line sharply rising to the predetermined maximum torque at which driving force shut-off occurs. At the other extreme stripped threads on the fastener may result in a spinning of the workpiece for a long period of time at a very low torque until a thread mismatch occurs at which time the torque will rise sharply to the maximum value. Both of these operations appear satisfactory if maximum torque exerted during the operation is the only quality control criteria employed.
This situation immediately suggests that the time of the torquing operation be measured and employed as a quality control factor along with maximum torque. However, the time required to achieve maximum torque during the torquing operation is first of all dependent upon the initial state of the threaded fastener members and unless a high measure of care is exercised in preparing the fasteners for automatic assembly, there will be a substantial variation in the time required to attain maximum predetermined torque using automatic equipment, with different sets of the same fasteners. Additionally, a threaded fastener may bind slightly during a torquing operation yet still attain a good final set after sufficient torque is exerted, less than the maximum torque, to overcome this bind. The torque-time curve of this fastener will be characterized by a gradual increase in torque until the bind occurs, a sharp increase in torque at that time, a sharp drop-off of torque after the bind is cleared, and finally a gradual rise to the maximum predetermined torque.
I have determined that while the time required to perform the torquing operation is a relevant factor in evaluating its quality, a much more precise evaluation of quality is attained by a measurement of the time during a torquing operation when the instantaneous torque being applied exceeds the previous peak torque applied during the operation. That measurement might be termed the "positive torque-time". If a slight bind is encountered in setting the fastener, the time after the bind, while the torque first decreases and then increases to the previous maximum value, is not measured. I found that this criteria of "positive torque-time" is highly useful in evaluating the quality of a torquing operation.